A fraction collector typically refers to an apparatus that is positioned in the outlet flow stream of a liquid chromatography system and used to collect portions of the system flow into separate collection vessels such as sample tubes or vials. Each collected portion is referred to as a fraction. Each fraction is obtained by collecting the entire liquid chromatography system flow starting at a specific time and continuing for a time window of fixed duration. Alternatively, the collection of each fraction may be initiated at the start of detection of a corresponding compound in the liquid chromatography system flow. In general, the collection of each fraction starts at a different time and the durations of the collected fractions are typically different.
A conventional hardware configuration for a fraction collector includes a diverter valve that, in one state, directs the liquid chromatography system flow to a waste channel and, in a second state, directs the liquid chromatography system flow to a collection tube or dispensing needle. As used herein, “collection tube” and “dispensing needle” are used synonymously and refer to a structure having a fluid channel through which a liquid flows from the diverter valve to a collection vessel. The dispensing needle generally is in the form of a flexible tube or other conduit that extends from the diverter valve and terminates at the other end as a dispensing needle tip which dispenses liquid into the collection vessel.
Typically, multiple collection vessels are available and the collection of a particular fraction is preceded by automated movement of the collection tube so that the dispensing needle is positioned at the opening of a corresponding collection vessel. To begin collecting a fraction, the diverter valve is actuated so that the system flow of the liquid chromatography system is diverted through the dispensing needle to the appropriate collection vessel instead of passing through the waste channel. The size of droplets dispensed from the needle tip impacts the repeatability and accuracy of the volumes of the collected fractions. In addition, analyte concentration typically changes throughout the duration of a fraction collection event. Consequently, poor volume repeatability attributable to droplet volume negatively affects fraction concentration repeatability.